Thick dielectric electroluminescent displays (TDEL) provide a great advance in flat panel display technology. TDEL displays comprise a basic structure of a substrate upon which an electrically conductive film is deposited forming the first electrode. A thick film layer consisting of a ferroelectric material is then deposited on the electrically conductive film layer. A phosphor film is deposited on the thick film layer followed by an optically transparent but electrically conductive film to form the second electrode in the structure.
Various aspects of manufacturing TDEL displays are described in Applicant's U.S. Pat. Nos. 6,610,352, 6,589,674, 6,447,654, 6,617,782 and 5,432,015 and International Patent Applications PCT/CA01/01234 and PCT/CA00/00561. The disclosure of these aforementioned applications and issued patent are hereby incorporated by reference in their entirety into the present disclosure.
TDEL displays provide for several advantages over other types of flat panel displays including plasma displays (PDP), liquid crystal displays (LCD), thin film electroluminescent displays (TFEL), field emission displays (FED) and organic electroluminescent devices (OLED). For example, TDEL displays provide greater luminescence and greater resistance to dielectric breakdown as well as reduced operating voltage as compared to TFEL displays. This is primarily due to the high dielectric constant of the thick film dielectric materials used in TDEL displays which facilitates the use of thick layers while still facilitating an acceptably low display operating voltage. The thick film dielectric structure, when deposited on a ceramic or other heat resistant substrate, may withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased temperature tolerance facilitates annealing of subsequently deposited phosphor films to improve their luminosity and stability.
In the fabrication of a TDEL structure, sulfide phosphors are deposited in a hydrogen sulfide atmosphere or evolve hydrogen sulfide during deposition or subsequent thermal processing. The hydrogen sulfide or associated hydrogen may react chemically with the lower electrode films (typically gold) causing degradation by coalescing the gold into spheroids during phosphor deposition. It is believed that the coalescence is caused by the destruction of an oxide layer inherently present on the gold electrode, thereby increasing the surface tension of the film and causing the coalescence. Furthermore, hydrogen sulfide may reduce the metal oxides of the thick dielectric layer leading to the possibility of alloying with the gold lower electrode within the display.
It is therefore desirable to improve the stability of the lower electrodes present in a TDEL display to minimize any degradation thereof, especially during various steps involved in the production of the display such as phosphor deposition or thermal processing.